The present invention pertains to instruments, and the process of applying these instruments to the scribing of ferromagnetic material to refine magnetic domain size. It is especially concerned with laser scribing instrumentation and methods of using such instrumentation for the high speed scribing of ferromagnetic sheet.
The development of high permeability grain oriented silicon steel resulted in a significant reduction in core loss, especially at inductions greater than 1.5 T (15 KG). This reduction in loss has been achieved primarily by improvements in the degree of grain orientation. Separation of the components contributing to the overall core loss has shown that the improved losses obtained are due to a reduction in the hysteresis component of the core loss. Further loss reduction can be achieved by refining the 180.degree. domain wall spacing, which results in a lowering of the eddy current component of core loss.
Over the past several years techniques have been developed to reduce the domain wall spacing by changing the magnetostatic or the magnetoelastic energy in the sheet. Insulative coatings that apply a tensile stress parallel to the rolling direction have been effective in reducing the domain wall spacing and the core loss. Mechanical, or physical scribing transverse to the sheet rolling direction is another technique that has been found to be effective in reducing domain spacing and lowering the losses. The disadvantages of mechanical scribing are that the insulative coating is disturbed, and the space factor is decreased.
Efforts to obtain the advantages of scribing without the aforementioned disadvantages have centered around the use of pulsed laser scribing techniques. It is known that irradiation of an iron-silicon alloy by a laser pulse of sufficient power density can vaporize material at the alloy surface causing a pressure shock wave to travel through the alloy causing dislocations and twins (see A. H. Clauer et al, "Pulsed Laser Induced Deformation in an Fe-3 Wt. Pct Si Alloy," Metallurgical Transactions A, Vol. 8A, January 1977 pp. 119-125). This deformation, like the deformation produced by mechanical scribing, can be used to control domain spacing. In fact, pulsed lasers have been applied to grain oriented electromagnetic steel sheet to produce shock wave induced arrays of deformation, see, for example, U.S. Pat. No. 4,293,350 and French patent application No. 80/22231 published on Apr. 30, 1981 (Publication No. 2,468,191 and European patent application No. 0033878 A2).
These processes have produced a need for the development of instrumentation and processes capable of laser scribing at the high speeds necessary to make scribing of large lots or heats of ferromagnetic material practical on a high volume, high throughput basis. The present invention addresses these needs.
Applicant has proposed a process for laser scribing of ferromagnetic sheet in which means are provided for curving the sheet in an arc and then scribing the sheet while it is curved.
In the proposed process the curved sheet would be translated in a direction parallel to the axis about which it is curved and past a laser beam traveling substantially perpendicular to the sheet's axis of curvature, thereby scribing magnetic domain refining lines in the magnetic sheet.
Included in this invention is the instrumentation required to carry out this process. In accordance with this invention this instrumentation includes a means for curving the sheet to the desired radius of curvature and a means for translating said sheet under a laser beam. The laser beam is scanned across the path of travel of the curved sheet by a rotating optical means for deflecting the beam through the arc length defined by the curved sheet, and onto the curved sheet surface.
Preferably said rotating optical means for deflecting is a polygonal device whose angularly related faces repeatedly reflect the laser light impinging on them as the device rotates, thereby directing the laser beam down to and across the curved sheet surface.
Preferably the means for curving the sheet are pairs of guide rolls having a concave and convex roll in each pair for curving the sheet.
Preferably the laser beam has been passed through a focusing lens prior to its being reflected.
The aforementioned and other aspects of the present invention will become more apparent upon examination of the drawings, which are briefly described below, in conjunction with the detailed description of the invention.